A hydraulic drive of the aforementioned type is disclosed in, for example, German Patent Application P 38 27 365.9-15.
In this drive mechanism, a hydraulic motor designed as an axial-piston motor is provided as the power drive, with a conventional follow-up control valve being included for the motion control, namely, direction of rotation and angular velocity. This valve operates with electrically controllable desired value presetting and mechanical actual value acknowledgment of the indicated dynamic parameters. For desired value introduction, a stepping motor or an AC motor is provided which, in turn, can be activated in accordance with direction of rotation and rotary velocity by output signals of an electronic control unit constituting an output stage of an NC or CNC machine control unit.
The electrical control motor, the follow-up control valve and the axial-piston motor are arranged, in this sequence, along the joint central longitudinal axis of the drive in side-by-side and, respectively, series relationship; feeding of pressure medium takes place to the individual linear cylinders of the axial-piston motor by way of a control cam fixedly mounted to the housing and revolving with the rotor of the axial-piston motor. As seen along the central longitudinal axis, this control cam is arranged "between" the follow-up control valve and the drive section of the hydraulic motor, and the rotor of the hydraulic motor is rotatably supported with its output shaft on a housing end section by two inclined ball bearings in order to obtain a maximally large effective axial length of the bearing, required at the high output power of the hydraulic motor, and yet to make do with small axial dimensions.
The drive mechanism according to the aforementioned patent application is somewhat disadvantageous by virtue of the necessary large total structural length, in spite of these insofar quite suitable structural features and additional aspects, such as the specifically provided type of coupling of the output shaft of the electric control motor, permitting the necessary axial relative motions, to a desired value presetting spindle of the follow-up control valve, and the coupling, accommodated within the rotor shaft, of the actual position value aknowledgment spindle of the follow-up control valve to the rotor of the axial-piston motor. Such length is linked also with relatively large lengths of the liquid columns within the drive mechanism and a concomitant loss of rigidity of the drive unit as a whole.